OSPF — area X range for ABR Type-3 summary aggregation

Routing
Advanced
14 min
3 steps
Cisco IOS-style

R1 advertises three /24 loopbacks (10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24) into area 1. Without summarisation, the ABR projects three Type-3 summaries into area 0. Configure area 1 range 10.0.0.0 255.255.0.0 on the ABR and watch the three /24s collapse into one /16 in area 0.

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Scenario

area <id> range <network> <mask> is OSPF's ABR-side aggregation knob. RFC 2328 §12.4.3: - When the ABR generates Type-3 summaries OUT of area <id>, every prefix that falls within <network>/<mask> is suppressed. - One summary Type-3 covering <network>/<mask> is emitted in its place. - Cost = max of contributing components (Cisco default; cost N overrides). Optional not-advertise keyword: suppress the components AND the summary — effectively a black-hole filter at the area boundary. In this lab three /24s under a /16 cover get aggregated. Verify with show ip route on the area-0 router (R3): one /16 entry instead of three /24s.

Topology

3 × router · Cisco IOS-style

R1 — router
ABR — router
R3 — router

What you will do

On R3, run show ip route ospf. Without aggregation you should see THREE inter-area routes (O IA) — one per /24.
On the ABR, configure area 1 range 10.0.0.0 255.255.0.0 under router ospf 1. This tells the ABR to aggregate area-1's 10.0.0.0/16 cover to a single Type-3 in area 0.
Back on R3, re-run show ip route ospf. The three /24s should be gone, replaced by one O IA 10.0.0.0/16 entry.

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